1. Field of the Invention
The present invention relates to an Orthogonal Frequency-Division Multiplexing (OFDM) cellular system, and more particularly, to a cell search method in an OFDM cellular system, a frame transmission method thereof, and a forward link frame structure thereof.
2. Description of the Related Art
Wideband Code Division Multiple Access (WCDMA) systems of the 3rd Generation Partnership Project (3GPP) use a total of 512 long Pseudo-Noise (PN) scrambling codes in order to identify base stations of a forward link. That is, adjacent base stations in a WCDMA system use a unique long PN scrambling code as a scrambling code of forward link channels. When a mobile station is turned on, the mobile station must acquire system timing of an initial cell and a long PN scrambling code identifier (ID) (it is also called a cell ID) of the initial cell. This process is called a mobile station's cell search process. The initial cell is determined according to a location of the mobile station when the mobile station is turned on, and generally indicates a cell of a base station corresponding to the greatest one of signal components of the base stations, which are included in a forward link reception signal of the mobile station. The system timing indicates slot sync or a frame boundary.
In a WCDMA system, in order to easily perform the mobile station's cell search process, the 512 long PN scrambling codes are grouped into 64 code groups, and a Primary Synchronization Channel (PSC) and a Secondary Synchronization Channel (SSC) are included in a forward link channel. The PSC is used for a mobile station to acquire slot sync, and the SSC is used for the mobile station to acquire a 10-msec frame boundary and a long PN scrambling code group ID.
The mobile station's cell search process in a WCDMA system is accomplished in 3 steps. In the first step, a mobile station acquires slot sync using a PSC. In the WCDMA system, the 10-msec frame includes 15 slots. Each base station transmits a PSC in every 10 msec frame. The same PSC is used for the 15 slots, and all base stations use the same PSC. In the first step, the mobile station acquires slot sync using a matching filter suitable for the PSC.
In the second step, a long PN scrambling code group ID and a 10-msec frame boundary are acquired using the slot sync (i.e., slot timing information) acquired in the first step and a SSC.
In the third step, a long PN scrambling code ID corresponding to a long PN scrambling code used by the initial cell is acquired using a common pilot channel code correlator based on the 10-msec frame boundary and the long PN scrambling code group ID that were acquired in the second step. That is, since 8 long PN scrambling codes are mapped to a single long PN scrambling code group, in the third step, the mobile station calculates a correlation value of each of the 8 long PN scrambling codes and detects the long PN scrambling code ID used in the initial cell based on the calculated result. In WCDMA, a sync channel consists of a PSC and a SSC, and the PSC, the SSC, a common pilot channel, and other data channels are multiplexed in a Code Division Multiplexing (CDM) method based on a time domain direct sequence spread spectrum.
Recently, in the 3GPP, an OFDM-based wireless transmission technology standardization is being established as a part of 3rd Generation Long Term Evolution (3G-LTE) to compensate for disadvantages of WCDMA. The sync channel & common pilot channel structure and the mobile station's cell search process used in WCDMA are suitable for Direct Sequence Code Division Multiple Access (DS-CDMA) but cannot be applied to an OFDM forward link. Thus, a forward link sync channel & common pilot channel structure and a mobile station's cell search method are required in an OFDM cellular system.